A study now in progress is examining the mechanism of sodium retention and ascites formation in rats with ligation of the common bile duct. Micropuncture measurements have demonstrated thus far that fractional sodium and water reabsorption by the proximal convoluted tubule is significantly increased. There is, in addition, a redistribution of glomerular filtration, with a reduction in surface nephron GFR but preservation of whole kidney GFR. The colloid oncotic pressure in postglomerular capillaries is low, indicating that an oncotic pressure driving force is not the mechanism of increased fractional reabsorption. Further studies will be carried out to measure hydrostatic pressure in the surface tubules and capillaries, in order to determine whether a change in pressure gradient may be responsible for the enhancement of sodium reabsorption by the surface nephrons. In another project, we are studying the ionic species of phosphate which is reabsorbed by the proximal convoluted tubule. In microperfusion studies, using solutions containing either H2PO minus 4, or HPO equals 4, measured the change in pH of the perfusion solution in vivo, by means of a microantimony electrode. We have found that the pH of the H2PO minus 4 solution rises in the proximal tubule, whereas that of the HPO equals 4, solution falls. The findings thus far suggest that phosphate is reabsorbed only in the H2PO minus 4 form. We plan to measure in similar experiments reabsorption of radioactive-labelled H2PO minus 4 and HPO equals 4 in separate solutions. We will then investigate the role of parathyroid hormone in the reabsorption of the two forms of phosphate and the interrelation between hydrogen ion secretion and phosphate reabsorption.